Linking old-growth forest composition, structure, fire … FEATURE: SUSTAINABILITY ON THE U.S./MEXICO BORDER Linking old-growth forest composition, structure, fire history, climate

SPECIAL FEATURE: SUSTAINABILITY ON THE U.S./MEXICO BORDER

Linking old-growth forest composition, structure, fire history,climate and land-use in the mountains of northern Mexico

CITLALI CORTES MONTANO,1,4,� PETER Z. FULE,1 DONALD A. FALK,2

JOSE VILLANUEVA-DIAZ,3 AND LARISSA L. YOCOM1

1School of Forestry, Northern Arizona University, P.O. Box 15017, Flagstaff, Arizona 86011 USA2School of Natural Resources and the Environment, University of Arizona, 325 Biological Sciences East, Tucson, Arizona 85721 USA

3Centro Nacional de Investigacion Disciplinaria en Relacion Agua, Suelo, Planta, Atmosfera-Instituto Nacional de InvestigacionesForestales Agrıcolas y Pecuarias, Km 6.5 Margen Derecha Canal de Sacramento, Gomez Palacio, Durango, CP 35140 Mexico

Citation: Cortes Montano, C., P. Z. Fule, D. A. Falk, J. Villanueva-Dıaz, and L. L. Yocom. 2012. Linking old-growth forest

composition, structure, fire history, climate and land-use in the mountains of northern Mexico. Ecosphere 3(11):106.

http://dx.doi.org/10.1890/ES12-00161.1

Abstract. Old-growth forests are biologically and ecologically valuable systems that are disappearing

worldwide at a rapid rate. Mexico still holds large areas covered by temperate forests in the mountains of the

Sierra Madre Occidental, but few of these retain old-growth characteristics. We studied four sites with

remnant old-growth forests in Mesa de las Guacamayas, a site in the Sierra Madre Occidental in

northwestern Chihuahua, to assess their composition, structure, and age characteristics. Overstory tree

densities and basal areas at each site were based on measurements of all trees .1.3 m tall. The overstory was

dominated by large Pinus durangensis, P. strobiformis, and Pseudotsuga menziesii (270–335 trees ha�1, basal area

24–42 m2 ha�1), with a subcanopy formed mostly of oaks. This species composition, combined with the lack

of vertical structural development, and thus of fuel ladders, suggests that these forests are relatively resistant

to severe wildfire. We evaluated forest attributes in the context of local fire regimes and regional climatic

patterns, and found that frequent disturbance by surface fires has been part of the study sites’ histories for at

least 250 years. While climate was a driver of fire regimes historically in this mountain range, humans appear

to have played a role in interruptions of the fire regime in the second half of the 20th century. Age

distributions showed recruitment to the canopy over ;250 years, while fires in the four sites recurred every

6–12 years. Temporary interruption of the fire regime in the mid-20th century at three sites was associated

with increased tree establishment, especially by broadleaved species. One site had an uninterrupted fire

regime and showed continuous tree establishment, consistent with the self-reinforcing role of frequent fire in

regulating live and dead fuel loads. Remnant old-growth forests such as those we sampled are becoming

increasingly rare in the Sierra Madre Occidental. The biodiversity and ecological processes that they support

are highly threatened and their conservation must be made a priority in the U.S.-Mexico borderlands.

Key words: old growth; pine-oak forests; Rhynchopsitta pachyrhyncha; Sierra Madre Occidental; Special Feature:

Sustainability on the U.S./Mexico Border.

Received 5 June 2012; revised 12 August 2012; accepted 16 August 2012; final version received 3 October 2012;

published 16 November 2012. Corresponding Editor: F. Biondi.

Copyright: � 2012 Cortes Montano et al. This is an open-access article distributed under the terms of the Creative

Commons Attribution License, which permits restricted use, distribution, and reproduction in any medium, provided

the original author and sources are credited.4 Present address: Tecnologico de Monterrey, Legado para la Sostenibilidad, Monterrey, Nuevo Leon, CP 64849 Mexico.

� E-mail: [email protected]

v www.esajournals.org 1 November 2012 v Volume 3(11) v Article 106

INTRODUCTION

Old-growth forests (OGFs) differ from second-ary forests in a broad range of ecological andbiological characteristics (Wirth et al. 2009). Forexample, they often have more diverse plantcommunities (Burrascano et al. 2009, D’Amato etal. 2009), increased stratification complexity, andhigher amounts of snags, logs, and large livetrees (Binkley et al. 2007). Old-growth forestsprovide key habitat for species that are not foundin secondary or harvested forests (Lammertink etal. 1998, Miller and Chambers 2007) and act assinks in terrestrial carbon dynamics (Carey et al.2001, Luyssaert et al. 2008, Keeton et al. 2010).They also can provide information regardingprocesses that may affect ecosystems over time,such as climate, fire, and other disturbances(Swetnam 1993, Swetnam and Baisan 2003,Kaufmann et al. 2007, Fule et al. 2012). However,the area covered by OGFs has decreased dra-matically worldwide in the past century (Bouch-er et al. 2009, Wirth et al. 2009). By 2009 it wasestimated that only 23% of the world’s forestsremained intact, with most of the remnant old-growth concentrated in boreal and tropicallatitudes (Wirth et al. 2009). Mexico has notescaped the global trends of accelerated forestcover loss and degradation (Challenger et al.2009, Sanchez Colon et al. 2009). Between the1970s and 2002, Mexico’s temperate forest coverdecreased by 25% and became increasinglyfragmented, mostly due to agricultural clearing(Sanchez Colon et al. 2009).

The Sierra Madre Occidental (SMOc) supportsthe largest continuous area of temperate tosubtropical forests in Mexico (Challenger andSoberon 2008). It runs in a north-south directionover approximately 1200 km of the country’sNorthwest, varying in width from 200 to 400 km.The SMOc is one of the least studied bioticprovinces in Mexico, due to its remoteness,rugged terrain, difficult access, and a long historyof social instability (Felger and Wilson 1994).Despite the limited information about its biolog-ical and ecological systems, it has been estimatedthat temperate forests of the SMOc have a plantendemism rate of 70%, the highest identified forall vegetation types in Mexico (Felger et al. 1997).Its temperate forests also offer unique opportu-nities to study systems with maintained fire

frequencies that in some cases have remainedunaltered into the 21st century (Fule et al. 2011,Fule et al. 2012).

Forests with uninterrupted fire regimes can bemore resistant to severe fire and are moreheterogeneous than those in which fires havebeen suppressed (Stephens and Fule 2005). Whilefire suppression policies have been in place inMexico since the early 1900s (Rodrıguez Trejo1999), loose enforcement and relatively sparseresources—compared to the United States—haveallowed historical fire regimes to continue up tothe present in some remote sites of the SMOc(Heyerdahl and Alvarado 2003, Stephens et al.2008, Fule et al. 2011, Fule et al. 2012). Previousstudies in the SMOc suggest a strong relationshipbetween fire occurrence and large-scale climaticpatterns, such as the El Nino Southern Oscilla-tion. Fire years coincide with dry years and areoften preceded by wet years that promoteaccumulation of fine fuels (Heyerdahl andAlvarado 2003, Fule et al. 2005, Cerano Paredeset al. 2010).

The natural conditions of forests in themountains of the SMOc in Chihuahua are closelyassociated with centuries of continuous humanuse and management. Prior to the arrival ofEuropeans in the 16th and 17th centuries, theseforests were part of the territories of variousindigenous groups (Lartigue 1983, Challenger1998). Evidence from trincheras (retaining wallsbuilt with rocks) suggests that humans haveoccupied the sierras of northern Chihuahua sincepre-Hispanic times (Herold 1965, Howard andGriffiths 1966). While indigenous people in theSMOc continue to use fire and other forestmanagement practices (LaRochelle and Berkes2003, Fule et al. 2011), commercial forestry hasbeen the most important agent of human-induced change in the SMOc since the early1900s (Challenger et al. 2009). To date, Chihua-hua remains one of the most important timber-producing states in Mexico (CONAFOR 2009).

By the end of the 20th century, changes in ageand structural characteristics in forests of theSMOc were so dramatic that two differentstudies (Escalante 1996, Lammertink et al. 1998)reported that only ;1% of the area once coveredby OGFs remained in the region. The ImperialWoodpecker, Mexican Grizzly Bear, and MexicanWolf were extirpated from the SMOc in the 20th


SPECIAL FEATURE: SUSTAINABILITY ON THE U.S./MEXICO BORDER CORTES MONTANO ET AL.

century, due in part to habitat loss and fragmen-tation, increased human presence and hunting(Challenger 1998). Thick-billed Parrots, calledguacamayas in Spanish, have been extirpatedfrom other mountain ranges in Mexico. Breedingpopulations are found in the mountains of theSMOc, and they are legally recognized as anendangered species (SEMARNAT 2010). Thick-billed Parrots favor old-growth habitat fornesting and breeding (Monterrubio-Rico andEnkerlin-Hoeflich 2004), and thus they areespecially susceptible to changes in forest struc-ture to the extent that the recent decline in theirpopulations has been attributed to habitat reduc-tion (Lammertink et al. 1998, Monterrubio-Ricoand Enkerlin-Hoeflich 2004).

Unlogged sites provide a unique opportunityto study interactions among forest development,fire, and large-scale climatic processes in OGFs.We studied four old-growth sites at Mesa de lasGuacamayas in northwestern Mexico, measuringforest structure, composition, age distribution,tree regeneration, surface and canopy fuels, andthe characteristics of large snags. We used thesedata, together with fire history information froma companion study (Fule et al. 2012), to answerthe following questions: (1) What are theattributes of the structure and floristic composi-

tion of the overstory, regeneration, and fuelloads? (2) What are the interactions of fire,regional climatic variability, and human-inducedland-use changes across a ;250 year timeline inthe landscape represented by the four studysites?

METHODS

Study areaThe Mesa de las Guacamayas (MDG) moun-

tain range is located at the northern end of theSMOc in northwestern Chihuahua, Mexico (Fig.1). It is part of Ejido Cinco de Mayo, a communalholding of ;25,000 ha of forests and grasslandscreated through the allocation of federal land bya presidential decree executed in 1972 (RAN2010). Large concessions in the region weregranted to various lumbering operations in thefirst half of the 20th century (Palomares Pena1991), but we were unable to confirm if theforests of MDG were included in these. In the1960s, at least one sawmill was established in thesite, operating until ejido formation in the 1970s.The ejido continued to log its more accessibleforests as recently as the late 1990s (Campos,personal communication). Only one family lives atMDG year-round, and current uses include light

Fig. 1. Location of the Mesa de las Guacamayas study area in the state of Chihuahua, Mexico. Study site names

and codes are: MP, Mesa Prieta; PS, Mesa Prieta Sur; RT, Rincon de las Tinajas; and AB, El Abeto.



grazing by cattle and equines.Conifer species that form the overstory in the

sampled sites are in the Cupressaceae (Juniperusdeppeana Steud. and Cupressus arizonica Greene)and Pinaceae (Abies sp., Pinus durangensis Martı-nez, P. strobiformis Engelm. (synonymous with P.ayacahuite var. brachyptera Shaw), and Pseudotsugamenziesii (Mirb.) Franco). Broadleaved species inthe four sites belong to four families: Robinianeomexicana A. Gray (Fabaceae); Quercus coccolo-bifolia Trel., Q. durifolia Seemen, Q. gambelii Nutt.,Q. grisea Liebm., Q. mcvaughii Spellenb., Q.sideroxyla Hum. & Bonpl. and Q. viminea Trel.(Fagaceae); Fraxinus velutina Torr. (Oleaceae); andPrunus sp. (Rosaceae).

Parent material in Mesa de las Guacamayas isof volcanic origin, formed mostly of rhyoliticignimbrites (Ferrari et al. 2007). Predominantsoils are Phaeozems and Regosols, with Lithosolspresent in small pockets (Unidad Forestal CasasGrandes-Babıcora 1999). Weather records for theregion are sparse and incomplete; the nearestweather station is Guapoca (located ;90 km tothe SE of the site at 1260 m of altitude). Data forthe period 1961–1998 show that mean annualvalues were 584.6 mm for precipitation, 15.98Cfor temperature, 6.38C for minimum temperatureand 25.68C for mean maximum temperature(CSM N.A., Estaciones climatologicas del sistemaCLICOM. Comision Nacional del Agua, Subdir-eccion General Tecnica, Coordinacion del Servi-cio Meteorologico).

Field methodsWe searched the study area on foot, looking for

forest stands dominated by Pinus spp. andPseudotsuga menziesii with no signs of harvesting(Fig. 2). We identified four such sites: Mesa Prieta(MP), Mesa Prieta Sur (PS), Rincon de las Tinajas(RT) and El Abeto (AB) (Fig. 1). We establishedten plots on a 1003 100 m sampling grid (N¼ 40plots) at each study site, on steep, north-facingslopes (Table 1). MP and PS were accessible onlyby foot, AB was at the end of a logging road andRT was on the side of a secondary road.

We used circular fixed-area plots located ateach grid point to obtain data about thestructure, composition and age of the overstory,and the regeneration stratum. Plots for overstorymeasurements were 200 m2 (7.98 m radius) and40 m2 for nested regeneration plots (3.57 m

radius). We marked each plot center permanentlyand recorded elevation, slope, and aspect.

We identified tree species in the field usingregional field guides for Pinus (Farjon et al. 2000)and Quercus (Spellenberg 2001). We collectedbotanical specimens for all species and depositeda full set of our collections (M. Joe and C. Cortes2009) at ASC, the Deaver Herbarium of NorthernArizona University. We considered Robinia neo-mexicana as a tree since it reached tree height andwas part of the subcanopy stratum. We recordedthe species of each tree or snag (for somedecaying snags we could record only the genus),and diameter at breast height (DBH, 1.3 m) of all(live and dead) trees and snags .1.3 m tall ineach 200 m2 plot. We also recorded total treeheight and canopy base height, and conditionclass of each stem using the nine-class systemdescribed by Thomas (1979). We tallied allsaplings or shrubs ,1.30 m tall, in each 40 m2

subplot using three height classes (1–40 cm; 41–80 cm, .80 cm). We used increment borers tocore all trees .30 cm DBH as well as a 10%subsample of randomly selected smaller trees ineach plot. Coring height was at 10 cm aboveground level to minimize age variation to coringheight. If trees were too small to core (,10 cmDBH), we cut a stem section at 10 cm aboveground level.

To assess fuel loads at the four sites we usedthe method described by Brown (1974), estab-lishing 15-m planar transects in a randomdirection from each plot center (N ¼ 40). Wetallied woody fuels in the first 5 m of the transectusing standard size classes: 1-hour timelag fuels(0–0.62 cm diameter), 10-hour timelag fuels(0.63–2.53 cm diameter), and 100-hour timelagfuels (2.54–7.61 cm). We measured all dead andlive 1000-hour timelag fuels (.7.61 cm at thenarrowest point), and the depth in cm of theorganic (duff and litter) layers every 5 m. Werecorded the presence or absence of canopy coverby vertical projection every 1 m along thetransect. We collected and crossdated 157 partialcross-sections from fire-scarred trees on the foursites and used them for fire history analyses (Fuleet al. 2012).

Laboratory methods and data analysisBotanists at ASC determined the identity of

botanical specimens. Some specimens were sent



to other herbaria for identification or verification

due to the lack of a recognized scientific flora for

the region. We calculated basal area (BA, m2

ha�1) and tree density (number of trees ha�1) for

all living and dead trees by species, and

regeneration density per species group per height

class. Given the high BAs of Pinus durangensis, P.

strobiformis and Pseudotsuga menziesii, we treated

Table 1. Physical and biological characteristics at the four sampling sites at Mesa de las Guacamayas, Chihuahua.

Site� Latitude (N) Longitude (W) Slope (%)Elevation(m asl)

Tree speciesrichness Establishment�

Indicatorspecies§

MP 308300 1088320 59.8 2375 2.6 1793 PIST(20–72) (2250–2550) (1, 4)

PS 308290 1088320 46.2 2409 2.4 1735 PSME(4–76) (2350–2447) (1, 5)

RT 308330 1088380 59.4 2467 2.0 1770 PIDU(26–90) (2377–2735) (1, 3)

AB 308320 1088370 63.4 2476 3.2 1766 ...(49–73) (2399–2617) (1, 5)

Note: Mean values are in boldface; minimum and maximum are in parenthesis.�Site abbreviations are: MP, Mesa Prieta; PS, Mesa Prieta Sur; RT, Rincon de las Tinajas; and AB, El Abeto.� Establishment is that of the oldest dated tree (year A.D.).§Indicator species from Indicator Species Analysis results (*P , 0.05). Abbreviations are: PIST, Pinus strobiformis; PSME,

Pseudotsuga menziesii; PIDU, Pinus durangensis.

Fig. 2. Old-growth forest at the Mesa Prieta Sur (MP) study site in Mesa de las Guacamayas. Left: Typical

canopy structure of live standing trees and large snags; large logs on forest floor. Right: Fire-scarred pines on the

foreground, subcanopy of broadleaved trees, a clump of younger trees to the left, and large dead fuels on the

ground.



them individually. We grouped the remainingspecies into three categories: Quercus spp., OtherConifers (Cupressus arizonica, Juniperus deppeanaand Abies sp.), and Other Broadleaved species(Fraxinus velutina, Prunus sp. and Robinia neo-mexicana). We relativized BA by plot in order toequalize weights for inter-plot analyses (McCuneand Grace 2002). Information regarding snagdensity, an important component of old-growthand Thick-billed Parrot habitat, will be presentedin a forthcoming paper.

Boxplots and scatterplots indicated that the BAdata were not normally distributed, so we usedmethods amenable to non-normally distributeddatasets to assess the composition of the over-story among the four sites. Non Metric Multidi-mensional Scaling (NMDS) is recommended forthe analysis of non-normally distributed ecolog-ical community data (McCune and Grace 2002).It reduces the dimensions of a distance matrixand derives an optimal solution to the relation-ships of species distribution in plots. We pro-duced an ordination with the NMDS function inPC-ORD (McCune and Mefford 2006), using therelativized BAs of the six groups and Sørensensimilarity indexes to create a distance matrixbased on three individual species and threemulti-species groups (N ¼ 39 plots, because oneplot was eliminated from the analyses due to lackof tree cover). We used a second matrix withenvironmental variables (for example, number offires at each site, elevation, fuel loads) to assesstheir correlations to the axis scores of theordination. We rotated the graphic to make thedisplayed vector (correlation .0.45) parallel tothe axis (McCune and Grace 2002). We conduct-ed 250 runs with the real data and 250 MonteCarlo randomized runs to ensure that the axesextracted by the ordination were not a result ofchance. We used random starting configurations,a maximum of 500 iterations per run and astability criterion of 0.00001. Once a final solutionwas achieved, its stress value was compared torandom solutions generated through MonteCarlo tests. Stress values for NMDS ordinationsare considered acceptable at ,20; and instabilityvalues are optimal at ,10�4 (McCune and Grace2002).

To assess the significance of the differences inspecies composition among the four sites weused the PERMANOVA function in the vegan

package (Oksanen et al. 2011) for R (R Develop-ment Core Team 2010). Permutational ANOVA(PERMANOVA) is a multivariate test analog tothe univariate ANOVA. It can be used with non-normally distributed data since it makes nodistributional assumptions (Anderson 2001). Itstest statistic is a pseudo-F, calculated throughpermutations under an assumed H0 that the dataare not responding to a particular groupingeffect, such as treatment or site (Anderson2001). We conducted paired PERMANOVA teststo explore differences in composition amongpairs of sites, since there were statisticallysignificant differences in species compositionamong the four sites. We estimated P-valuesusing 9999 permutations, which is well over theminimum of 5000 permutations recommendedfor a , 0.01 (Anderson 2001).

To determine characteristic species for each sitewe used the Indicator Species Analysis (ISA)function in PC-ORD. We used a dataset thatincluded all tree species with relative BA . 2.5%.These were Cupressus arizonica, Pinus durangensis,P. strobiformis, Prunus sp., Pseudotsuga menziesii,Quercus coccolobifolia, Q. gambelii and Q. mcvaugh-ii. We considered a species to be a strongindicator for a site when its indicator value was.25% and significant (P , 0.05) (Dufrene andLegendre 1997).

In order to understand regeneration dynamicsand to create an age distribution for the foursites, we reconstructed age distributions from thecollected cores. We mounted and sanded coresand sections with increasingly finer sandpapergrits until individual cells were visible with amicroscope. We used a master tree-ring chronol-ogy for the area (Villanueva et al., unpublished ) tocrossdate each core or section. We used a pithlocator to establish the number of rings to centerfor cores that missed the pith (Applequist 1958).Pith locators are transparent overlays of concen-tric circles with curvatures that vary as a functionof the distance to the pith. Since we did not haveenough dateable cores and sections to match thesix species and groups used for the analysesdescribed above, we grouped the cores into twobroad categories (Conifers or Broadleaved) toanalyze the age data.

We used the equations in Brown (1974) tocalculate fuel loads from the planar transects.Since we were unable to find coefficients for



specific gravity and average squared diametersof fine fuel particles for SMOc species, we usedthe coefficients for Pinus ponderosa, a speciesclosely related to P. durangensis (Gernandt et al.2009).

We used a dataset created by a companionstudy (Fule et al. 2012) to model fire dates andfire-climate relations. A tree-ring based climaticreconstruction of the Palmer Drought SeverityIndex (PDSI) provided information to establishrelationships between historical climatic patternsin the region, tree establishment and recruitmentto the overstory, and fire histories. The PalmerDrought Severity Index is a hydroclimatic mea-sure of the extent and intensity of drought at theregional scale (Alley 1984). We used a tree-ringbased reconstruction of PDSI developed by Cooket al. (2004), which includes four grid points closeto our research area (points 105, 106 and 120,121). Grid point 121 is located in NW Chihuahua,but its PDSI reconstruction ends in 1990, so weused grid point 120, located in SW New Mexico,USA. Grid point 120 is the closest point to ourresearch area with a record that extends into the21st century, and is highly correlated (Pearson’s r¼ 0.94) with grid point 121.

RESULTS

Forest structure and compositionPinus durangensis, P. strobiformis and Pseudotsu-

ga menziesii had the highest basal areas in thefour sites (Table 2). The species with the largesttrees was Pseudotsuga menziesii, followed byPinus durangensis and P. strobiformis (Fig. 3). Thegroups Other Conifers, Quercus spp. and Broad-leaved had higher numbers in the smallestdiameter classes and very few individuals .40cm DBH. Tree density was highest in the AB site(1195 trees ha�1), mostly due to the abundance ofRobinia neomexicana; and lowest at MP (380 treesha�1). Basal area was lowest at site MP (26.8 m2

ha�1), and highest at PS (43.6 m2 ha�1), driven bythe large Pseudotsuga menziesii trees found there.

The final NMDS ordination (Fig. 4) had threeaxes and was achieved with a final stress of13.013 and 0.06 instability. The ordination ex-plained 91.6% of the variability in BA andsimilarity (Axis 1 ¼ 30.7%, Axis 2 ¼ 38.6%, andAxis 3 ¼ 22.5%). Correlations between speciesand the ordination axes showed that Axis 1 wasdriven by a transition in dominance from Pinusstrobiformis to Pseudotsuga menziesii and Axis 2 bydominance of Pinus durangensis. The vectoroverlay suggested that Axis 2 was also influ-enced by the number of fires in the past 60 years,which was greater at site RT. Indicator SpeciesAnalysis results (all P , 0.05) showed that Pinusdurangensis was a good indicator for RT (Indica-tor Value, IV¼57%), Pinus strobiformis for MP (IV¼ 47%) and Pseudotsuga menziesii for PS (IV ¼42%). The ISA results supported plot groupings

Table 2. Structural characteristics (number of trees and basal area by species and species groups)� of the overstory

(all trees .1.3 m tall) by site at Mesa de las Guacamayas.

Site Characteristic PIDU PIST PSME OTH CON QUERCUS OTH BL Total

MP TPH 20 130 30 90 110 ... 380(0–50) (0–400) (0–100) (0–900) (0–450) ... (0–900)

BA 3.7 11.3 7.8 0.9 3.1 ... 26.8(0–20.2) (0–41.4) (0–58.5) (0–9.3) (0–9.2) ... (0–58.5)

PS TPH 60 55 150 70 360 25 720(0–600) (0–550) (0–1500) (0–700) (0–3600) (0–250) (0–3600)

BA 10.1 1.4 29.9 0.5 1.7 0.01 43.6(0–37.0) (0–5.4) (0–71.6) (0–2.8) (0–8.6) (0–0.04) (0–71.6)

RT TPH 250 5 25 ... 125 50 455(100–2500) (0–50) (0–250) ... (0–1250) (0–500) (0–2500)

BA 26.9 0.0 1.85 ... 0.5 0.4 29.7(4.7–50.1) (0–0.02) (0–8.6) ... (0–2.4) (0–3.73) (0–50.1)

AB TPH 25 90 120 75 295 590 1195(0–250) (0–900) (0–1200) (0–750) (0–2950) (0–5900) (0–5900)

BA 6.4 4.2 17.8 3.1 2.8 0.6 34.9(0–27.5) (0–22.0) (0–49.7) (0–22.1) (0–9.6) (0–4.8) (0–49.7)

Notes: Site codes are as in Table 1. Mean values are in boldface; minimum and maximum are in parenthesis. TPH is number oftrees per hectare, BA is basal area (m2 ha�1).

�Species abbreviations are: PIDU, Pinus durangensis; PIST, Pinus strobiformis; PSME, Pseudotsuga menziesii; OTH CON, otherconifers; QUERCUS, Quercus spp.; OTH BL, other broadleaved species.



based on dominance and indicator values ofthose three species. The lack of indicator speciesfor AB is due to high variation in the compositionof the overstory in this site. The most diversestratum at the four sites was the subcanopy,formed of Quercus spp. and other broadleavedspecies.

Paired PERMANOVA tests showed that theplant community at RT was statistically signifi-cantly different from all other sites (Bonferroniadjusted a¼ 0.008; MP-RT pseudo-F¼ 3.728; PS-RT pseudo-F ¼ 4.781 RT-AB pseudo-F ¼ 4.358).Pinus durangensis comprised 90.7% of the totalBA at RT. Pinus strobiformis and Pseudotsugamenziesii were dominant species at the other sites(MP, PS and AB), followed by Quercus spp. in MPand PS, and Other Conifers (Cupressus arizonicaand Juniperus deppeana) in AB.

Tree age and demography, fuels,fire and climate

We were able to crossdate 63% of the total

collected cores and sections, 69 from conifers and18 from broadleaved species. The oldest trees inthe PS, RT, and AB sites were from the specieswith highest basal area values: Pinus durangensis,P. strobiformis, and Pseudotsuga menziesii (Fig. 5).Quercus sp., included in the Broadleaved group,comprised the oldest age group at MP. However,by the end of the 18th century, Pinus andPseudotsuga had established in all the sites. Theoldest dateable trees established in the mid-18thcentury. Other trees established earlier than thisdate, but the cores could not be dated to pith andwere excluded from the analyses. The excludedcores were collected from two Pseudotsugamenziesii in PS and one Pinus durangensis in AB,with inner ring dates of 1735, 1747, and 1766,showing that two study sites had older trees thanthose included in the age distribution. Establish-ment of conifers and broadleaved species in theearly 20th century was observed at PS and RT,and all sites showed a pulse of recruitmentstarting at or after the 1950s. The age distribution

Fig. 3. Diameter distribution of live trees in the four study sites at Mesa de las Guacamayas. The x-axis

indicates the upper end of each diameter category. The top panel shows the diameter distributions of all conifer

species and the bottom panel shows those of Quercus spp. and other broadleaved species. Site labels are as in Fig.

1.



showed that relatively few broadleaved trees,including Quercus spp., established between 1750and 1900 A.D. A pulse of recruitment of broad-leaved species was observed in the late 1990s andearly 2000s.

The regeneration stratum was dominated byQuercus spp. at PS, RT, and AB (Fig. 6). Pinusspp. was abundant in the 0–40 cm class at MP.Robinia neomexicana, included in the Broadleavedgroup, was a major component of the .40–80 cmheight class, but it dwindled dramatically in thenext class.

Average litter depth was deepest at RT (3.1 cm)and shallowest in MP (1.5 cm). Average duffdepth was deepest at MP (2.6 cm) and shallowestat PS (1.0 cm). Coarse woody debris loads werehighest at AB (19.17 Mg ha�1) and lowest at PS(13.18 Mg ha�1) (Table 3). Average canopy covervalues for the four sites ranged between 51.4%(RT) and 60.7% (PS and AB) (Table 4). Pseudot-suga menziesii had the highest average height ofall trees in the canopy at MP, PS, and RT (MP:

22.9 m total, 5.1 m crown base, PS: 22.4 m total,6.6 m crown base, RT: 17.5 m total, 6.6 m crownbase) and Pinus durangensis at AB (25.0 m totalheight, 9.4 m crown base height).

Fires occurred frequently (mean fire intervals,10 yr) in the four sites before the mid-20thcentury (Fule et al. 2012). The dates of fires thatscarred �25% of the samples, a measure ofrelatively large fires, are shown in Fig. 5. Usingthis filter, 37 fires burned the four study sitesbetween 1733 and 2008, while 68 fires weredetected using all scarred trees for the sameperiod (a measure of smaller fires) (Fule et al.2012). A fire-free period began in 1945 at MP andPS, and 1953 at AB, and ;50 years later, thosesites burned at least once between 1989 and 2009.One site (RT) maintained an uninterrupted fireregime through the early summer of 2009 (Fule etal. 2012).

Regional hydroclimatic patterns, expressed byPDSI, were related inconsistently to temporalpatterns of tree ages and fire events in the last

Fig. 4. Non-metric multidimensional scaling ordination (stress¼ 13.01, instability¼ 0.05) of study sites at Mesa

de las Guacamayas based on relativized basal areas of overstory species. The ordination was rotated 208 to the

right to make the vector parallel to Axis 2. Total variance explained by the three axes was 91.7% of the variation in

the distance matrix. Axis 1 (30.7% of variation explained) is driven by a transition in abundance from Pinus

strobiformis to Pseudotsuga menziesii. Axis 2 (38.6% of variation explained) and was driven by abundance of Pinus

durangensis and fire frequency since 1950 A.D., as shown by the vector overlay. Site labels are as in Fig. 1.



250 years (Fig. 5). A weak climate signal wasobserved in post 1950s recruitment trends in allsites; otherwise we did not visually detectstrong associations between climate, fire fre-quency, and tree demography. There was anincrease in broadleaved tree recruitment in thesecond half of the 20th century, following the

fire-free period of 1950–1990 in PS and AB. Apulse of conifer establishment was observed inMP in the 1970s. This site had the lowest overallrecruitment among the study sites. In RT weobserved consistent conifer recruitmentthroughout the 20th century, with a pulse inthe 1930s.

Fig. 5. Climatic reconstructions, years of establishment of conifer and broadleaved species, and fire histories at

the four study sites in Mesa de las Guacamayas. (a) PDSI index reconstruction for point 120 in SW New Mexico

(Cook et al. 2004). (b–e) Age distributions and fire histories for each site. Dates are shown for fires that scarred

25% or more of the samples collected at the study sites in a companion study (Fule et al. 2012). Highlighted

decades (1950–1990) show fire interruption in MP, PS and AB. Site labels are as in Fig. 1.



DISCUSSION

Until the 1950s, fire frequencies of the four sitesat MDG resembled those of pre-European con-ditions of ponderosa pine forests in Arizona andNew Mexico (Fule et al. 1997, Swetnam andBaisan 2003) and other sites in northern Mexico(Fule and Covington 1997, Heyerdahl andAlvarado 2003, Fule et al. 2005, Fule et al.2012). Fire records at the four study sites showhigh synchrony, except for a ;50 year fireinterruption period in MP, PS, and AB. Firereturn intervals were ,10 years across the four

sites until the 1940s. Three sites underwent a fire-

free period that ended in the early 2000s. Only

one site (RT) maintained a 6.2 year mean fire

frequency into the 21st century, which makes it a

rare example of a relatively continuous fire

regime in southwestern North America.

Fire is still at work in the forests of Mesa de las

Guacamayas, since fires were recorded at least

once in all the sites since the late 1990s. Despite

alterations to the historical fire regimes in the

past 50 years, fuel loads in these sites remained

relatively low. Average fuel loads were compa-

rable to those of other unharvested sites in the

Fig. 6. Average density of seedlings and saplings in the regeneration stratum at the four study sites in Mesa de

las Guacamayas. Site labels are as in Fig. 1.

Table 3. Depth by site of forest floor fuels and woody debris loads by site at Mesa de las Guacamayas. Values are

expressed in logarithmically increasing standard time lag classes based on the time of each fuel size to reach

moisture equilibrium with ambient conditions.

Site Litter (cm) Duff (cm) 1 hr 10 hr 100 hr 1000 hr sound 1000 hr rotten Total

MP 1.5 2.6 0.29 1.02 5.86 6.25 ... 13.43(0.4–2.9) (0–9.6) (0–0.99) (0.20–2.82) (0–15.76) (0–34.56) ... (0–34.56)

PS 1.9 1.0 0.47 1.99 5.94 4.77 ... 13.18(0.4–3.8) (0.4–2.0) (0.09–1.32) (0.21–7.40) (0–30.89) (0–40.98) ... (0–40.98)

RT 3.1 2.6 0.17 1.60 2.47 8.12 5.35 17.73(1.1–5.1) (0.2–5.4) (0–0.36) (0.60–2.46) (0–9.39) (0–48.69) (0–35.29) (0–48.69)

AB 1.7 2.0 0.62 1.39 3.63 12.09 1.45 19.17(0.5–3.3) (0.4–3.9) (0.06–1.57) (0.20–3.38) (0–10.49) (0–70.88) (0–9.43) (0–70.88)

Notes: Site codes are as in Table 2. Mean values are in boldface; minimum and maximum values are in parentheses. Time lagclass values are expressed in megagrams per ha (Mg ha�1).



SMOc (Fule and Covington 1997, Stephens andFule 2005) and below the current average of 23.3Mg ha�1 for pine and pine-oak forests in Mexico(Rodrıguez Trejo 2008). The dominant overstorytrees, mostly in the Pinaceae, are tall and haverelatively high total height to crown base heightratios. This could be induced by self-pruning, bythe effect of frequent fires, or by both processes,which keep lower branches from reaching theforest floor and forming fuel ladders.

The tree-ring record of Mesa de las Guaca-mayas indicates that episodic drought has beenpart of the disturbance regime of these foreststhroughout the reconstructed period. The firehistories for the four sites show that fires tendedto occur during or following drought episodes(Fule et al. 2012), which are largely entrained bythe El Nino oscillations. However, the highsynchrony of fire occurrence at the four siteswas interrupted abruptly in 1945 at MP and PS,and in 1953 at AB. The initiation of this fire-freeperiod in those three sites coincided with the1950s regional drought observed in tree-ringrecords of the U.S. Southwest and northernMexico (Villanueva et al. 2009, Swetnam andBrown 2010).

In contrast to pre-European settlement fire-freeperiods of the southwestern U.S., which coincid-ed with wet periods (Swetnam and Brown 2010),fire interruption at MDG coincided with the1950s drought. This was the driest period of the20th century in the region (Grissino-Mayer andSwetnam 2000), suggesting that fire exclusionwas not related to wet conditions. Instead, wehypothesize that the fire-free period may havebeen due in part to logging and road buildingdriven by the holders of the concessions, and bythe introduction of livestock grazing. However,despite the ;60 year absence of fire, all sitesburned at least once in the 21st century and noneshowed evidence of widespread fire or pathogeninfestations.

Diversity in species composition commonlytranslates to diversity in tree architecture, whichconfers structural heterogeneity to the overstory.This attribute has been associated with distur-bance resistance of OGFs in northwestern Mexico(Stephens et al. 2008) and the U.S. Southwest(Binkley et al. 2007). The overstory at thesampling sites was heterogeneous, and differentspecies were dominant in each site. The species-rich subcanopy at the sample sites could play an

Table 4. Canopy fuel characteristics (number of trees and tree height by species and species groups) by site and

percent canopy cover by site at Mesa de las Guacamayas.

Site Characteristic PIDU PIST PSME OTH CON QUERCUS OTH BLCanopycover (%)

MP N 2 26 6 18 22 ...TH 12.2 12.1 22.9 5.3 4.1 ...

(11.5, 12.8) (1.4, 33.7) (4.9, 38.9) (1.6, 17.3) (1.3, 16.3) ... 54.0CBH 3.3 3.7 5.1 1.4 1.2 ... (6.7, 93.3)

(2.0, 4.6) (0.1, 13.3) (0, 12.1) (0, 3.9) (0.2, 3.6) ...CBH:TH 27.0% 30.6% 22.3% 26.4% 29.3% ...

PS N 12 11 29 14 71 5TH 16.6 9.5 22.4 2.9 2.5 1.8

(1.4, 33.7) (3.1, 20.0) (4.0, 48.3) (1.4, 6.0) (1.3, 11.0) (1.6, 2.3) 60.7CBH 7.8 4.1 6.6 0.4 0.8 0.4 (0, 80.0)

(0.3, 21.1) (0, 11.6) (0, 18.1) (0, 1.9) (0, 4.4) (0.1, 1.3)CBH:TH 47.0% 43.2% 29.5% 13.8% 32.0% 22.2%

RT N 43 1 5 ... 24 10TH 16.6 2.3 17.5 ... 3.1 2.6

(1.6, 43.4) (2.3, 2.3) (6.7, 38.1) ... (1.4, 10.4) (1.3, 8.3) 51.4CBH 9.5 0.9 6.6 ... 1.0 1.1 (6.7, 80.0)

(0.5, 32.6) (0.9, 0.9) (1.9, 10.1) ... (0, 3.7) (0, 3.7)CBH:TH 57.2% 39.1% 37.7% ... 32.3% 42.3%

AB N 3 18 22 16 59 118TH 25.0 6.4 12.5 6.4 3.5 2.1

(2.4, 38.5) (1.5, 33.2) (1.4, 37.7) (1.1, 22.0) (1.3, 13.8) (1.3, 16.3) 60.7CBH 9.4 2.0 2.8 1.2 1.1 0.9 (13.3, 100.0)

(0.1, 17.0) (0.1, 14.2) (0, 13.3) (0, 5.8) (0, 4.4) (0.2, 2.0)CBH:TH 37.6% 31.3% 22.4% 18.8% 31.4% 42.9%

Notes: Site and species codes are as in Table 2. Mean values are in boldface; minimum and maximum values are inparentheses. N is number of trees measured, TH is total height (m), CBH is crown base height (m).



important role providing key habitat for wildlife,as has been shown in forests of southernChihuahua (Miller and Chambers 2007).

Information regarding historical and currentignition sources is circumstantial at best. There isevidence that links the recent (;250 years)human history of the Mexico-U.S. borderlands(Seklecki et al. 1996) and fire incidence. However,the lack of documentation or other evidence ofthe role of humans makes these assertionsdifficult to prove.

Given the lack of information about ignitionsources, the uninterrupted fire regime of RTposes an intriguing question about the role ofhumans as agents of ecological processes. In ourview, this site represents an example of howhuman history is linked inextricably to fireregimes as both a top-down and a bottom-upfactor. The fire regime of MDG is comparable tothat of Pino Gordo, a remote site in the SMOcthat is managed as a multiple use landscape byRaramuri people (Fule et al. 2011). However, incontrast to Pino Gordo, decisions concerning themanagement of the landscape in MDG have beenmade by a succession of geographically distantstakeholders, including the federal government,timber concessionaires, foresters and even theejido itself. Boundary conflicts between EjidoCinco de Mayo and a neighboring landownercould be a partial cause of the site’s uninterrupt-ed fire history, maintained into the 21st century.This site was relatively closer to the valley and itshuman settlements, and thus nearer to landswhere fire is used as a management tool foractivities such as grass burning or agriculturalplot clearing.

Management implicationsThe forests of MDG are important biodiversity

reservoirs, providing habitat to endangeredspecies such as Thick-billed Parrots, and highlydiverse understory plant communities (CortesMontano 2011). These forests appear to maintaintheir structural characteristics and fuel loadsthrough frequent fires. Consequently, they couldbe used to set reference conditions for manage-ment of similar ecosystems in the SMOc and theSky Islands in Arizona and New Mexico.

The frequently burnt OGFs at Mesa de lasGuacamayas may be more resistant to distur-bances like severe fire and drought. The resis-

tance of the forests at the study site could bederived from the fire histories and the heteroge-neity of the landscape matrix in which they arefound. This contrasts with the extensive cata-strophic fires occurring currently in dense, fire-excluded forests of the western U.S. (Allen et al.2010, Williams et al. 2010). The forests at MDGare a potential source of organisms that are betteradapted to face the systemic changes expectedfrom climate change in the region (Williams et al.2010).

For almost one month in the late spring of 2012an estimated 13,500 ha of oak, pine-oak andmixed conifer forests burned in the MDG range.Satellite imagery and MODIS data points showedthat some of our sites may have been affected bythe fires. If this is confirmed and new studies arecarried out, the information presented in thispaper represents a baseline that sets the contextto understand forest dynamics in this region,including the effects of disturbances like fire.

The old-growth heritage of the SMOc urgentlyneeds to be incorporated into Mexico’s conserva-tion agenda. Their future is threatened by humanactivities and lack of protection in a situation thatparallels the early 1990s old-growth controversyof the Pacific Northwest in the U.S. and BritishColumbia in Canada (Johnson and Swanson2009). Uncertainty derived from climate changealso adds to the threats faced by these systems.Conservation of these forests should include firemanagement that incorporates frequent surfacefires, while structural and compositional com-plexity should be maintained in order to ensurehabitat quality for old-growth dependent species.

ACKNOWLEDGMENTS

Don Tomas in Tres Tumbas provided invaluablehelp as field guide. Don Teodoro Campos in Janosfacilitated obtaining permission from the ejido, andEjido Cinco de Mayo kindly let us work in their forests.Nelida Barajas Acosta and Jose Luis Garcıa Loya fromThe Nature Conservancy-Rancho El Uno helped withlogistics and field work. Isaac Bickford, Edgar BustillosDaniel, Julian Cerano Paredes, Walker Chancellor,Marco Cordoba Castillo, Nancy Duarte, Christen Irby,Marissa Joe, Fernando Morales, and Margarita MorenoHinojos helped with field work. Marissa Joe, TinaAyers, and Socorro Gonzalez helped with the organi-zation and determination of the botanical specimens.Chris McGlone helped with statistical analyses; KatieIreland, and Amanda Stan reviewed early versions of



this manuscript. Tony DeLuz at the Bilby ResearchCenter of NAU created Fig. 5. NSF grant DEB-0640351and CONACYT provided support during the doctoralstudies of the first author.

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Linking old-growth forest composition, structure, fire … FEATURE: SUSTAINABILITY ON THE U.S./MEXICO BORDER Linking old-growth forest composition, structure, fire history, climate